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Sorting disk I/O requests...Sorting disk I/O requests...Posted Oct 8, 2002 2:27 UTC (Tue) by tigerand (guest, #6150)In reply to: Sorting disk I/O requests... by axboe Parent article: A new deadline I/O scheduler This doesn't sound like it takes software raid (md) or filesystem I/O footprints into account. So if either of those two subsystems have any I/O scheduling type optimizations, then these optimizations are almost certainly going to clash with very unsatisfactory results. It sounds to me like this needs a heck of a lot more thought before it should be truely adopted. Even just sorting between reads and writes could clash with optimizations in some of the fancier journaling file systems. Delayed writes and the like could cloud the algorythm. There really needs to be a way to synchronize between file system code, low level block i/o code, and md code. For instance, I can see this scheme making a huge difference on a single disk with multiple file systems and maybe even a couple of chuckleheads dd'ing to a partition (or, god forbid, a user level file system like Coda or some databases that want to use raw partitions). In that case, a file system might want to wave off it's own optimizations in favor or this. But if it's a single file system on a disk situation, the file system code might know better, especially with it's advanced knowledge about what might be coming just behind this, but isn't in the queue yet. For instance, it knows that right now there are some data writes to accomplish, but right after that there's going to be some journal writes, so save up some pending journal writes just a tad, because there's a couple more just around the corner. This is an example of special knowledge, not a conflicting optimization. But you get the idea. Bottom line, this seems like a thing that belongs in the file systems, and at the low level is probably the wrong place to "fix" this.
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